Field of the Invention
The invention relates to the field of electronic reproduction technology and pertains to a method of adapting color values that have been produced for a first printing process to a second printing process so that the visual impression of the colors in both printing processes is the same.
In reproduction technology, printing originals for printed pages are produced that contain all the elements to be printed, such as texts, graphics and images. In the case of the electronic production of printing originals, these elements are present in the form of digital data. For an image, the data are produced, for example, by the image being scanned point-by-point and line-by-line in a scanner, each image point being broken down into color components and the color components being digitized. Images are usually broken down in a scanner into the color components red, green, and blue [R,G,B], that is to say, into the components of a three-dimensional color space. For the colored print, however, other color components are needed. In the case of four-color printing, these are the printing colors cyan, magenta, yellow, and black [C,M,Y,K], that is to say, the components of a four-dimensional color space. For such a purpose, the image data from the RGB color space of the scanner must be transformed into the CMYK color space of the printing process to be used.
Such color space transformations are needed in reproduction technology because all the devices and processes have their restrictions and special features in the representation and reproduction of the colors, and all the devices and processes have different such characteristics. For this reason, for various devices and processes such as scanners, monitors, proof output devices, printing processes, and so on, there are different color spaces that respectively describe the color characteristics of the device or process in an optimum way and that are referred to as device dependent color spaces.
In addition to the device dependent color spaces there are also device independent color spaces, which are based on the human visual characteristics of a standard observer, as referred to in the prior art. Such color spaces are, for example, the XYZ color space defined by the Commission Internationale d""Éclairage (CIE) standardization commission or the LAB color space that is derived therefrom, the LAB color space having made more progress in the technology. If one wishes to know whether two colors will be sensed by the human eye as the same or different, then the measurement of the XYZ or LAB color components is sufficient for such a purpose. The LAB color components form a color space with a lightness axis [L] and two color axes [A,B], which can be imagined in the plane of a color circle through whose center the lightness axis runs. The LAB color components are related to the XYZ color components through nonlinear conversion equations.
A device or process can be characterized in terms of its color characteristics by all the possible value combinations of the associated device dependent color space being assigned the LAB color components that a human sees in the case of the colors produced with these value combinations. For a printing process, the various CMYK value combinations respectively produce a different printed color. Using a color measurement instrument, the LAB components of the printed colors may be determined and assigned to the CMYK value combinations. Such an assignment, which sets the device dependent colors produced with a device or process in a relationship with a device independent color space (XYZ or LAB), is also referred to as a color profile, as an output color profile in the case of a printing process. The definition and data formats for color profiles have been standardized by the International Color Consortium (ICC) Specification ICC.1:1998-09. In an ICC color profile, the association between the color spaces in both directions is stored, for example, the association LAB=f1 (CMYK) and the inverted association CMYK=f2 (LAB).
The association defined by a color profile can be implemented by a look-up table. If, for example, the CMYK color components of a printing process are to be assigned the LAB color components, the look-up table must have a storage location for each possible value combination of the CMYK color components, in which location the associated LAB color components are stored. The simple association method has the disadvantage, however, that the look-up table can become very large. If each of the color components [C,M,Y,K] has been digitized with 8 bits, that is to say has 28=256 density steps, there are 2564=4,294,967,296 possible value combinations of the CMYK color components. The look-up table must, therefore, have 4,294,967,296 storage cells each with a word length of 3 bytes (one byte each for L,A,B). The look-up table therefore reaches a size of 12.3 gigabytes.
To reduce the size of the look-up table, a combination of look-up table and interpolation method is, therefore, used to describe a color profile and to implement a corresponding color space transformation. The associations for all the possible value combinations of the CMYK color components are not stored in the look-up table; only those for a relatively coarse, regular grid of reference points in the CMYK color space. The grid is formed by only each kth value being taken as a grid point in each component direction. For k=16, therefore, in each component each 16th value from the 256 possible values is taken as a grid point. Accordingly, in each component direction, the grid has 256/16=16 grid points, that is to say, 16xc3x9716xc3x9716xc3x9716=65,536 grid points for the entire CMYK color space. For each grid point, the associated components of the LAB color space are stored as reference points in the look-up table. For CMYK value combinations that lie between the grid points, the LAB values to be assigned are interpolated from the adjacent reference points. For the inverted assignment CMYK=f2 (LAB), a grid of 16xc3x9716xc3x9716=4096 grid points, for example, is formed in the LAB color space, and the associated CMYK values are stored as reference points in the look-up table.
The assignments given in the color profiles between device dependent color spaces and a device independent color space (e.g., LAB) can be used for the color space transformation between the device dependent color spaces, so that, for example, the color values [C1,M1,Y1,K1] of a first printing process can be converted into the color values [C2,M2,Y2,K2] of a second printing process such that, according to the visual impression, the second print has the same colors as the first print. FIGS. 1a and 1b show a color space transformation for such a printing process adaptation according to the prior art in a block diagram. In FIG. 1a, a first color space transformation (1) from the color values [C1,M1,Y1,K1] of the first printing process into LAB color values, and a second color space transformation (2) from the LAB color values into the color values [C2,M2,Y2,K2] of the second printing process are carried out one after another. The two color space transformations (1) and (2) can also be combined into an equivalent color space transformation (3), which assigns the color values [C1,M1,Y1,K1] and the color values [C2,M2,Y2,K2] directly to one another (FIG. 1b). Because in each case, through the device independent LAB intermediate color space, the color values [C1,M1,Y1,K1] and [C2,M2,Y2,K2] that result in the same LAB color values are assigned to one another, the associated printing colors in the two printing processes are sensed as visually the same within the printing color gamut. However, one disadvantage of such a method is that the black build-up (as it is referred to in the art) of the first printing process is lost. Black build-up is understood to mean the composition of printed colors with respect to their proportion of the black printing ink K. In particular, the aim is for pure black colors, such as blocks of text, for example, to be built up only with the printing ink K, that is to say contain no CMY components. With the above-described method according to the prior art, it is not possible to achieve the situation where pure black colors that, in the first printing process, are built up only with the printing ink K are also built up only with the printing ink K in the second printing process. In general, based upon visual equivalence, that is to say, the same LAB color values, in the second printing process mixed colors are assigned that, although they predominantly contain proportions of the printing ink K, also contain CMY components. Among other things, following the printing process adaptation, such a process leads to black texts and lines being given colored edges in the event of register errors in the printing machine.
In addition, in the process according to the prior art, it is not ensured either that the lightness curve in black or gray colors, as set in the first printing process, will be reproduced correctly in the second printing process following the adaptation. The reason is that the associated black or gray colors of the second printing process contain additional CMY components, and that the proportion of K is formed in accordance with the lightness curve of the second printing process, which was set during the creation of the color profile of the second printing process.
A further disadvantage of the above-described method is that the black build-up of the first printing process is lost in the chromatic colors. Because, in the four-color printing system, it is possible for the same color to be printed with many different CMYK value combinations, the system is ambiguous, and it is possible to choose whether gray colors and dark colors are to be printed with a relatively high proportion of the black printing ink K and correspondingly low proportions of the colored printing inks [C,M,Y] or with a low proportion of K and correspondingly higher proportions of [C,M,Y]. The decision is made with prior art methods, such as under-color removal (UCR) or gray-component reduction (GCR). According to the conventional method of printing process adaptation described, the decision that was made for the color values [C1,M1,Y1,K1] of the first printing process is not transferred into the associated color values [C2,M2,Y2,K2] of the second printing process. Instead, the associated color values [C2,M2,Y2,K2] are formed in accordance with the black build-up of the second printing process, which was set when the color profile of the second printing process was created.
It is accordingly an object of the invention to provide a method of adapting a printing process while maintaining the black build-up that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that avoids the aforementioned restrictions and disadvantages and specifies a method of printing process adaptation from a first printing process with the color values [C1,M1,Y1,K1] to a second printing process with the color values [C2,M2,Y2,K2] that operates based upon given color profiles for the two printing processes, and with both the visually sensed colors and the black build-up of the first printing process being maintained.
With the foregoing and other objects in view, there is provided, in accordance with the invention, A method for producing a color space transformation, including the steps of converting color values of a first printing process into color values of a second printing process to substantially transfer black build-up of the first printing process into the second printing process and to make a visual impression of printed colors in both printing processes be substantially the same by characterizing color reproduction characteristics of the first printing process with a first color profile specifying an association between device dependent color values of the first printing process and color values of a device independent color space, characterizing color reproduction characteristics of the second printing process with a second color profile specifying an association between device dependent color values of the second printing process and color values of a device independent color space, determining a first lightness curve from the first color profile, determining a second lightness curve from the second color profile, calculating an inverted lightness curve from the second lightness curve, linking the inverted lightness curve and the first lightness curve to form a first transformation function, determining second transformation functions from the first color profile, determining third transformation functions from the second color profile, calculating inverted transformation functions from the third transformation functions, linking the inverted transformation functions and the second transformation functions to form fourth transformation functions, and joining the fourth transformation functions to the first transformation function to form a color space transformation between color values of the first printed process and color values of the second printing process.
In accordance with another mode of the invention, before inverting the second lightness curve, the second lightness curve is corrected to be monotonic.
In accordance with a further mode of the invention, before inverting the third transformation functions, the third transformation functions is corrected to be monotonic.
In accordance with an added mode of the invention, the color space transformation is corrected by determining the device independent color values from the first color profile for the device dependent color values of the first printing process, determining the device independent color values from the second color profile for the device dependent color values of the second printing process associated with the color space transformation, calculating corrected color values from a deviation between the device independent color values determining corrected device dependent color values from the corrected color values, and replacing, in the color space transformation, the associated device dependent color values of the second printing process with the corrected device dependent color values.
In accordance with a concomitant mode of the invention, the correction of the color space transformation is repeatedly carried out until a mean deviation between the device independent color values falls below a threshold value.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of adapting a printing process while maintaining the black build-up, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.